With the onset of Forefront Threat Management Gateway (TMG) as the new version of ISA everybody is eager to hear how to migrate to this new version.
I just found a video that gives some guidance and a technet page that outlines on how to do it.
Thing with ISA 2006 was that it had to be installed on a 32 bit operating system to work. And TMG of course requires a 64 bit operating system. So an in place upgrade of ISA to TMG is not possible. It will ahve to be a move to other hardware or a re-install of an operating system on the existing machine after exporting settings from the old ISA installation.
I am planning my upgrade already, although of the two ISAs in my network one is on such old hardware that using any new OS on that hardware is impossible. But it is a good way to test a TMG to ISA-06 VPN tunnel.